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CRF means: Constant Rate Factor. It keeps an internal distortion metric in the encoding workflow (called "rate factor") below a threshold. But many details are part of the RF calculation, and not all represent your personal subjective quality impression perfectly. Yet, it is a much better one than PSNR and allows an easy adjustment of the quantization with some respect to the encoding complexity. But a few seemingly paradox results may happen. Heavier efforts do not always result in more efficient video stream code or visually more convenient pictures, exceptions from a general rule aren't impossible.
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